1. Field of the Invention
This invention relates to a display terminal which displays character images on a display screen or surface by scanning the display surface and detects the position of a light pen detecting field on the display surface, which is pointed to and sensed by a light pen.
2. Description of the Prior Art
The light pen has been used to have an operator point to a particular light pen detecting line which may be overlappingly displayed with a character dot pattern displayed on a display surface or screen of a display device of a raster scan type, such as cathode ray tube display device. Referring to FIGS. 7A and 7B showing one example of the prior technology, a first light pen detecting line shown by double circles is displayed in character columns N-1 and N in a light pen detecting field K, and a second light pen detecting line is displayed in character columns N+2, N+3, N+4, . . . in a light pen detecting field K+1. The intensity of the light pen detecting lines is high intensity so that a light sensor of the light pen generates a large sense pulse. When the operator wants to process a group of the characters in the light pen detecting field K, the operator positions the light pen on the light pen detecting line of the field K. It is assumed that the light pen is positioned at dot 70. The light sensor of the light pen generates a sense pulse 71 at To. During the display of the detecting line of the field K, a control device knows the column address, i.e. character addresses N-X . . . , N-1, N included in the detecting field K. The sense pulse 71A represents the sense pulse reaching or arriving at the control device after a minimum transmission delay time T1. The control device, receives the sense pulse 71A and judges the column address selected by the light pen from N-X, ..., N-1, and N. However, an unexpected additional delay time T2 could be generated, whereby the sense pulse 71 arrives at the control device after a long delay time T3 as shown by a pulse 71B. By the way, when the light pen is positioned on a dot 72 in the detecting line in the detecting field K+1, the sense pulse 73 is generated. It is assumed that pulse 73 arrives at the control device after a delay time T4. Therefore, there are two cases when the sense pulse arrives at the control device at time T5. One is when the sense pulse 71B from the detecting field K arrives and the other is when the sense pulse 73A from the detecting field K+1 arrives. Thus when the light pen is positioned at the dot 70 in detecting field K, the control device may misjudge the sense pulse 71B as the sense pulse 73A from detecting field K+1.
The prior technology solves the above misjudgment problem by measuring the expected maximum delay time of the sense pulse 71, as shown in FIG. 7B, and placing the additional space codes in columns N+2 and N+3, which fall into the maximum delay time. This approach, however, requires an additional two spaces, which do not display the character images, so that this approach reduces the number of the display character images on the display surface, and displays unnaturally long spaces in columns N+1, N+2 and N+3.
Japanese patent application 60-136526 discloses a method for compensating the transmission delay time to detect the actual position of the light pen in the detecting field. Before use of the display terminal, a reference dot at a known address on the display surface is detected by the light pen. An address represented by the detected signal from the light pen is compared with the known address. A value representing a difference between the known address and the detected address is stored in a memory. The value is used to compensate the detected address. The method solves the problem caused by a fixed transmission delay time of the detected signal. But, the transmission delay time could be variable due to variation of circuit parameters caused by operating temperature, a change of brightness of the cathode ray tube by the operator, voltage deviation of a power supply, etc.. The method disclosed in Japanese patent application 60-136526 cannot compensate for the variable transmission delay times.